Batman was trying to terrorize Joker with the Batplane, make the Joker defenseless by shooting Joker's men and scare Joker by shooting all around him, fear and intimidation are Batman's greatest weapons. Batman wanted to savor the moment and prolong Joker's agony. There would have been no satisfaction in killing Joker right away. But everything leading up to the killing would have been real satisfying. Everything seemed to be going so well. Batman though he had made Joker defenseless, Batman thought he had everything under control, and I'm sure he thought Joker would be screaming for mercy, trying to run away, but Joker stood his ground fearlessly with no fear of death and had another surprise prepared for Batman - an obscenely long phallic gun with an explosive shell.